The millimeter wave frequency range (30-300 GHz) has advantages for many communication and radar applications because of the wide bandwidth and high angular resolution for a given antenna size. The development of millimeter wave (mm wave) systems has been hindered, however, by the immature technology of components such as power amplifiers. Currently available systems use traveling wave tubes (TWTs), which are expensive, require high voltage power supplies, and are vulnerable to single point failure. An alternative is the use of solid state Impact devices, which are expensive and difficult to tune. Because the technologies related to both TWTs and Impact devices are relatively mature, order of magnitude improvements in performance and cost seem unlikely.
Gallium arsenide (GaAs) transistors, either heterojunction bipolar transistors (HBTs) or high electron mobility transistors (HEMTs), offer promise for mm wave power amplification. Unlike Impact diodes, these are three-terminal devices that are much easier to control. However, HBTs and HEMTs suffer from fundamental limitations in terms of the amount of power that can be generated from each discrete device. Typically, the amount of power generated by a device is inversely proportional to the square of the frequency (i.e., p.about.1/f.sup.2) For devices operating at the extremely high frequencies in the mm wave range, the maximum power for a single transistor is low (on the order of 200-250 mW at 44 GHz, for example). Therefore, the combined output of many transistors is needed to provide power for practical systems, which can require from several to hundreds of watts.
Conventional approaches to power combining at mm wave frequencies use bulky waveguides and/or precision transmission lines. The lossiness of passive networks using these components imposes an upper limit on the maximum power attainable, which falls significantly short of the requirements for many systems. What is needed is mm wave power amplifier that can achieve an order of magnitude (or greater) improvement in performance and cost compared with conventional technologies.